In many applications, it is desired to acquire spectral images of an object. Spectral imaging implies that the spectral content of an image of the object is extracted and incident electro-magnetic radiation relating to multiple narrow ranges of wavelengths is detected. The spectral imaging may for instance be used in analysis of imaged objects, such as for determination whether a substance having a specific spectral profile is present in the object.
The terms multi-spectral imaging and hyperspectral imaging are often used. These terms do not have established definitions, but typically multi-spectral imaging refers to spectral imaging using a plurality of discrete wavelength bands, whereas hyperspectral imaging refers to imaging narrow spectral wavelength bands over a continuous spectral range. Hyperspectral imaging may also often use a larger number of spectral bands than what is used in multi-spectral imaging.
With advances to semiconductor fabrication, it is now possible to produce image sensors based on complementary metal-oxide-semiconductor (CMOS) technology, wherein a substrate comprising an array of photo-sensitive areas is integrated with filters for selectively transmitting light to the photo-sensitive areas.
The integrated filters may define different wavelength bands associated with different photo-sensitive areas. For instance, spectral bands may be associated with different rows of photo-sensitive areas, such that when a line-scan is performed an image of high spectral and spatial resolution may be acquired. Alternatively, a mosaic configuration of wavelength bands may be arranged on the array of photo-sensitive areas, such that a set of photo-sensitive areas is associated with a plurality of filters and may together acquire spectral information of a portion of an object. An image sensor having a mosaic configuration of filters may be used for acquiring an image with a spectral resolution in a single exposure (snapshot). However, a spatial resolution of the image will be limited to a combined size of the set of photo-sensitive areas associated with the plurality of filters. Also, other set-ups of an image sensor for acquiring images having spectral resolution may be used.
A filter defining a wavelength detected by a photo-sensitive area may define a central wavelength and a bandwidth around the central wavelength, which will be transmitted to the photo-sensitive area with which the filter is associated. A bandwidth of the transmitted light may be defined by a full width at half maximum (FWHM), i.e., a range of wavelengths for which at least half the intensity of the maximum intensity of transmitted light for the central wavelength will be transmitted to the photo-sensitive area.
However, the bandwidth may depend on materials used in the filter, such that the FWHM value may be larger for longer wavelengths. This implies that the selective detection of wavelengths may be different longer wavelengths. In some applications, the filters may provide a similar bandwidth for all wavelengths that are to be detected by the image sensor. Therefore, the image sensor may be configured in a flexible manner for defining characteristics of light transmitted to the photo-sensitive areas of the image sensor.